The present invention relates generally to data and voice communications over digital subscriber lines and, more particularly, to a method and apparatus for filtering signals in a splitterless asymmetric digital subscriber line (ADSL) system.
Asymmetric Digital Subscriber Line (ADSL) refers to a new modem technology that allows existing twisted pair telephone lines to be converted into a high-performance access path for multimedia and high-speed data communications. An ADSL circuit connects an ADSL modem on each end of the twisted pair telephone line, creating three information channelsxe2x80x94a high speed downstream (central office to end user) channel, a medium speed upstream (end user to central office) channel, and a POTS (xe2x80x9cPlain Old Telephone Servicexe2x80x9d) channel. The POTS channel is separated from the ADSL modem by filters, thus guaranteeing uninterrupted POTS, even if the ADSL circuit fails.
While description is provided in terms of the POTS channel, telephone voice communications signals, telephone instruments, and the like for the benefit of familiarity, it should be understood that telephone equipment and signals need not be limited to voice communications, but may also include other technologies, for example equipment and signals compatible with regular telephone lines, such as facsimiles machines, voiceband modems (for example, V.90 modems), answering machines, and the like.
Two variants of ADSL systems are available today xe2x80x94full-rate ADSL in accordance with the T1E1.413 or ITU G.992.1 standards and xe2x80x9csplitterlessxe2x80x9d ADSL defined by the ITU G.992.2 standard. Full-rate ADSL uses POTS splitters to separate the POTS channel from the ADSL data signals. A POTS splitter is installed at each end of the line and includes a lowpass filter for separating out POTS telephone voice communication signals and a highpass filter for separating out data communication signals.
The POTS splitter divides the subscriber line into two separate twisted pairsxe2x80x94one for data communication (ADSL) and one for telephone voice communication signals (POTS). As a result, the existing two-wire internal house telephone wiring is not usable for ADSL. New wiring must be installed from the splitter to the modem, resulting in increased installation cost.
Splitterless ADSL can be installed without the need for additional home wiring. In this case, the ADSL modem includes a high-pass filter that rejects the POTS telephone voice communication signal, while every telephone instrument in the house is connected to the telephone line through a low-pass filter that rejects the ADSL data signals.
When only one telephone instrument is used, the well-known design of LC filters is adequate to implement the required low pass filter. However, in real life, several telephone instruments are usually connected to each telephone line, each of them in an on-hook or off-hook state. In xe2x80x9csplitterlessxe2x80x9d ADSL, the result of such a configuration is that several lowpass filters are connected in parallel on the same telephone line. Certain electrical properties of a telephone instrument, for example its input impedance, depends on the operational state or hookswitch condition (e.g., whether the telephone instrument hookswitch is on-hook or off-hook). Hookswitch condition can refer to the on-hook or off-hook states of the hookswitch or the transitions of the hookswitch between these states. As a result, certain filter characteristics, for example the frequency response, of a low pass filter connected to a telephone instrument will change when the telephone changes its state.
In reality, the behavior is much more complicated. A low pass filter connected to an on-hook telephone has zero impedance at 4 kHz, which will produce distortion in another telephone instrument, should it happen to be off-hook. Several on-hook telephones connected in parallel will create several resonance frequencies (Universal ADSL Technical Group Contribution, Document # [TG/98-121]; xe2x80x9cPreliminary Report of the POTS Filter and Power Reduction Ad-hoc;xe2x80x9d Bob Beeman; Redmond, Wash.; Apr. 14, 1998; pp. 1-9).
FIG. 1 is a block diagram illustrating a splitterless ADSL system of the prior art. Customer premises equipment (CPE) 101 is coupled to central office (CO) 102 by digital subscriber line (DSL) 103. CPE 101 includes a highpass filter 104, ADSL modem 107, computer 108, lowpass filters 105 and 106, and telephone instruments 109 and 110. Computer 108 is coupled to ADSL modem 107, which is coupled to highpass filter 104, which is coupled to DSL 103. Telephone instrument 109 is coupled to lowpass filter 105, which is coupled to DSL 103. Telephone instrument 110 is coupled to lowpass filter 106, which is coupled to DSL 103.
CO 102 includes a POTS (xe2x80x9cplain old telephone servicexe2x80x9d) splitter 111, ADSL modem 112, data switch 113, voice switch 114, data network 115, and voice network 116. DSL 103 is coupled to POTS splitter 111, which is coupled to voice switch 114 and ADSL modem 112. Voice switch 114 is coupled to voice network 116. ADSL modem 112 is coupled to data switch 113, which is coupled to data network 115.
Voice communications passing through voice switch 114 are passed through POTS splitter 111 and applied to DSL 103 as baseband signals. Data communications passing through data switch 113 are modulated at a frequency range higher than that of the baseband POTS signals and passed through POTS splitter 111 and applied to DSL 103. Since the data communications are transmitted at a different frequency range than the voice communications, frequency-division-multiplexing (FDM) allows simultaneous transmission of both voice communications (POTS) and data communications over a single DSL 103.
Since data communications are suitably processed by ADSL modem 107 and computer 108, while voice communications are intended for telephone instruments 109 and 110, highpass filter 104 and lowpass filters 105 and 106 provide selective filtering of the voice and data communications. Highpass filter 104 passes the higher frequency data communications to ADSL modem 107 and computer 108, while blocking the lower frequency baseband voice communications. Lowpass filters 105 and 106 pass the lower frequency baseband voice communications to telephone instruments 109 and 110, respectively, while blocking the higher frequency data communications.
Unfortunately, lowpass filters 105 and 106 exhibit a deficiency that can adversely affect the performance of the ADSL system. The frequency response of lowpass filters 105 and 106 changes based on the status of telephone instruments 109 and 110, respectively. For example, while lowpass filter 105 might properly differentiate between voice communications and data communications when telephone instrument 109 is off-hook (e.g, when telephone instrument 109 is in use), the electrical characteristics of lowpass filter 105 are altered when telephone instrument 109 is returned to its on-hook state (e.g., when the user hangs up). This change in the electrical characteristics of lowpass filter 105 can cause interference with the data communications between ADSL modem 107 of CPE 101 and ADSL modem 112 of CO 102. Thus, a circuit is needed that will allow telephone instruments, such as telephone instruments 109 and 110, to change between their off-hook and on-hook states without adversely affecting ongoing data communications over DSL 103.
FIG. 2 is a block diagram illustrating a lowpass filter and telephone instrument of the prior art. Lowpass filter 201 is coupled to telephone instrument 202. Telephone instrument 202 includes a load 203, which exhibits a load impedance. Load 203 is coupled in series with hookswitch 204. When telephone instrument 202 is off-hook, hookswitch 204 is closed, coupling load 203 to lowpass filter 201. However, when telephone instrument 202 is on-hook, hookswitch 204 is open, disconnecting load 203 from lowpass filter 201. Thus, the electrical characteristics (e.g., the frequency response) of lowpass filter 201 are affected by load 203. Therefore, a circuit is needed to compensate for the undesirable interaction between load 203 and lowpass filter 201.
The present invention is directed to a method and apparatus for filtering ADSL signals. The filtering technique provided by the invention avoids interference when telephone instruments coupled to a DSL switch between the off-hook and on-hook states. One embodiment of the invention provides a variable inductance in series with a simple lowpass filter used to block high-frequency data communications signals. The variable inductance is controlled by an off-hook detector that senses the status of the telephone instrument hookswitch.
Another embodiment of the invention includes a parallel combination of a resistance element and a switch in series with the DSL to provide attenuation and control the overall electrical characteristics of the filter assembly and telephone instrument. Another embodiment of the invention includes an inductive element with a saturable core in series with the DSL. The saturable character of the inductor core results in the inductive element having different electrical properties depending on whether the telephone instrument is on-hook or off-hook.
Thus, the invention reduces interference to data communications that can otherwise occur in a splitterless ADSL system if a telephone instrument changes hookswitch state.